As is the case for other genomes and metagenomes, complete nucleotide sequences of bacteriophages and archaeviruses have become increasingly numerous and require robust annotation tools. We present here the first version of a phage ontology, PhiGO, which should contribute to more informational annotation of gene products in phage and prophage sequences. PhiGO uses the Gene Ontology schema, the de facto standard for describing knowledge about gene products across many databases.